Why do you think that this efficiency is low? What is the maximum theoretically possible efficiency? The maximum theoretically possible efficiency would be 100%.

I think the efficiency is low because we have a number of .817.The efficiency of any machine depends upon the amount of friction and air resistance present. A frictionless machine would have an efficiency of 100%. Here we have .817 for efficiency, so we must have a lot friction or air resistance.

You used g = 9.80665, which is indeed the standard acceleration due to gravity; however, g is more commonly approximated to 9.81 since the actual value of g varies greatly depending on location. It is not a serious problem though.